Chapter 3: Modeling Data in the Organization

1. Chapter 3:Modeling Data in the Organization

2. Business Rules • Statements that define or constrain some aspect of the business • Assert business structure • Control/influence business behavior • Expressed in terms familiar to end users • Automated through DBMS software

3. Examples of Business rules • A student may register for a section of a course only if he or she has successfully completed the prerequisites for that course. • A preferred customer qualifies for a 10 percent discount, unless he has an overdue account balance.

4. Entity-Relationship Model (E/R) The Entity-Relationship model (ER) is a high-level description of the structure of the DB. The Entity-Relationship Diagram (ERD) is a graphical model for representing the conceptual model for the data. A E/R models the DB using three element types: - Entities - Attributes - Relationships

5. E-R Model Constructs • Entities: • Entity instance–person, place, object, event, concept (often corresponds to a row in a table) • Entity Type–collection of entities (often corresponds to a table) • Relationships: • Relationship instance–link between entities (corresponds to primary key-foreign key equivalencies in related tables) • Relationship type–category of relationship…link between entity types • Attribute–property or characteristic of an entity or relationship type (often corresponds to a field in a table)

6. Sample E-R Diagram

7. Entity symbols Attribute symbols A special entity that is also a relationship Relationship symbols Basic E-R notation Relationship degrees specify number of entity types involved Relationship cardinalities specify how many of each entity type is allowed

8. What Should an Entity Be? • SHOULD BE: • An object that will have many instances in the database • An object that will be composed of multiple attributes • An object that we are trying to model • SHOULD NOT BE: • A user of the database system • An output of the database system (e.g., a report)

9. Example of inappropriate entities System output System user Inappropriate entities Appropriate entities

10. Entity types • Person: EMPLOYEE, STUDENT, PATIENT • Place: STORE, WAREHOUSE, STATE • Object: MACHINE, BUILDING, AUTOMOBILE • Event: SALES, REGISTRATION, RENEWAL • Concept: ACCOUNT, COURSE, WORK CENTER

11. Attributes • Attribute–property or characteristic of an entity or relationship type • Classifications of attributes: • Required versus Optional Attributes • Simple versus Composite Attribute • Single-Valued versus Multivalued Attribute • Stored versus Derived Attributes • Identifier Attributes

12. Identifiers (Keys) • Identifier (Key)–an attribute (or combination of attributes) that uniquely identifies individual instances of an entity type • Candidate Identifier–an attribute that could be a key…satisfies the requirements for being an identifier

13. Characteristics of Identifiers • Will not change in value • Will not be null • No intelligent identifiers (e.g., containing locations or people that might change)

14. The identifier is boldfaced and underlined Simple and composite identifier attributes

15. Simple & Composite Attributes Simple attribute is an attribute that have a single value with an independent existence. e.g. student_no ,sex Composite attribute is an attribute composed of multiple distinct components, each with an independent existence. e.g. name , phone_number An attribute broken into component parts

16. Single-valued & Multi-valued Attributes Single-valued attribute is an attribute that holds a single value for a single entity. • e.g. staff_no , DOB, gender Multi-valued attribute is an attribute that may hold multiple values, of the same type, for a single entity. e.g. skill , degree, address Multivalued an employee can have more than one skill

17. Derived Attributes Derived attribute is an attribute that represents a value that is derived from the value of a related attribute. e.g. Age of a person can be calculated from person’s date of birth and present date. Derived from date employed and current date

18. Simple example of time-stamping This attribute is both multivalued and composite

19. More on Relationships • Relationship Types vs. Relationship Instances • The relationship type is modeled as lines between entity types…the instance is between specific entity instances • Relationships can have attributes • These describe features pertaining to the association between the entities in the relationship • Two entities can have more than one type of relationship between them (multiple relationships) • Associative Entity–combination of relationship and entity

20. Figure 3-10 Relationship types and instances a) Relationship type b) Relationship instances

21. Degree of Relationships • Degree of a relationship is the number of entity types that participate in a relationship Unary Relationship Binary Relationship Ternary Relationship

22. Entities of two different types related to each other Entities of three different types related to each other One entity related to another of the same entity type Degree of relationships

23. Cardinality of Relationships • One-to-One • Each entity in the relationship will have exactly one related entity • One-to-Many • An entity on one side of the relationship can have many related entities, but an entity on the other side will have a maximum of one related entity • Many-to-Many • Entities on both sides of the relationship can have many related entities on the other side

24. Examples of relationships of different degrees Unary relationships

25. Examples of relationships of different degrees (cont.) Binary relationships

26. Cardinality Constraints • Cardinality Constraints—the number of instances of one entity that can or must be associated with each instance of another entity • Minimum Cardinality • If zero, then optional • If one or more, then mandatory • Maximum Cardinality • The maximum number

27. A patient history is recorded for one and only one patient A patient must have recorded at least one history, and can have many Examples of cardinality constraints a) Mandatory cardinalities

28. A project must be assigned to at least one employee, and may be assigned to many An employee can be assigned to any number of projects, or may not be assigned to any at all Examples of cardinality constraints (cont.) b) One optional, one mandatory

29. A person is married to at most one other person, or may not be married at all Examples of cardinality constraints (cont.) c) Optional cardinalities

30. Examples of multiple relationships a) Employees and departments Entities can be related to one another in more than one way

31. Examples of multiple relationships (cont.) b) Professors and courses (fixed lower limit constraint) Here, min cardinality constraint is 2

32. Figure 3-15a and 3-15b Multivalued attributes can be represented as relationships simple composite

33. Strong vs. Weak Entities, andIdentifying Relationships • Strong entities • exist independently of other types of entities • has its own unique identifier • identifier underlined with single line • Weak entity • dependent on a strong entity (identifying owner)…cannot exist on its own • does not have a unique identifier (only a partial identifier) • partial identifier underlined with double line • entity box has double line • Identifying relationship • links strong entities to weak entities

34. Identifying relationship (Figure 3-5) Strong entity Weak entity

35. Associative Entities • An entity–has attributes • A relationship–links entities together • When should a relationship with attributes instead be an associative entity? • All relationships for the associative entity should be many • The associative entity could have meaning independent of the other entities • The associative entity preferably has a unique identifier, and should also have other attributes • The associative entity may participate in other relationships other than the entities of the associated relationship • Ternary relationships should be converted to associative entities

36. A binary relationship with an attribute(a) Here, the date completed attribute pertains specifically to the employee’s completion of a course…it is an attribute of the relationship

37. An associative entity (CERTIFICATE) (b) Associative entity is like a relationship with an attribute, but it is also considered to be an entity in its own right Note that the many-to-many cardinality between entities in (a) has been replaced by two one-to-many relationships with the associative entity

38. An associative entity – bill of materials structure

39. Ternary relationship as an associative entity

40. Microsoft Visio Notation for Pine Valley Furniture E-R diagram (Figure 3-22) Different modeling software tools may have different notation for the same constructs